This invention relates to solid state switching devices employing optical isolation. Depending on the application and power level, these devices can be characterized as solid state relays (SSR's) or analog switches. In the invention, field-effect transistor (FET) technology is combined with certain optoelectronic means alone or supplemented by electromagnetic means to achieve both superior switching characteristics and fabrication means compatible with low-cost mass production.
It is common in the electronics industry, particularly in modern telecommunications, test or computer equipment, to have control circuits operating at different voltage levels from load circuits. For example, it is common to operate 120 VAC industrial machines or 48 VDC telephone apparatus from 5 VDC computer logic.
As a result, the need arises to transfer data or control signals between such dissimilar voltage references without physically connecting the two circuits, i.e. with isolation.
In the evolving 1960's and 1970's state of the art, the isolation functions have had increasingly stringent criteria of miniaturization, long life under repetitive conditions, and low operating power. Satisfying a large part of these needs are four categories of isolated switching elements:
(a) Optical Couplers
These are generally low current (under 100 mA) elements employing an LED as a light emitter and a variety of semiconductor types as the sensor.
(b) Solid State Relays (SSR's) with Optical Isolation
These generally employ an internal optical coupler in combination with other semiconductor elements to switch power.
(c) Solid State Relays with Transformer Isolation
(d) Reed Relays
While the primary purpose of couplers is only to isolate, the purpose of reed relays and SSR's is not only to isolate but also to provide power gain.
Over the past ten years, as isolators and SSR's have achieved a degree of maturity and widespread acceptance, it has also become apparent that a variety of limitations exist.
For example, couplers generally are limited in current and voltage. They are usually limited to DC loads and are particularly susceptible to damage when switching inductive loads or when operating in an overload current mode. Furthermore, they exhibit a voltage offset of about one volt because of the use of bipolar photo-semiconductors. This means that voltages under one volt cannot be switched and that audio signals cannot be faithfully reproduced without the use of biasing.
While SSR's can offer considerable power gain and isolation, they are generally relegated to being designed for AC or DC operation, but not both. Also, an AC-load SSR employing a thyristor as the output element is subject to false latchup. In any event, such an SSR is not capable of being turned off in mid-cycle.
Reed relays experience difficulties with reactive loads and, although faster than conventional mechanical relays, are still slower than solid state switches. Also, reed relays are limited in the degree they can switch momentarily high currents or voltages. Finally, under very high switching rates, their mechanical life has a severe limitation.
Because of these limitations, increasing attention is being given to field effect transistors (FETs). Modern FETs offer the absence of significant voltage offset and thermal runaway effects, extremely high power gain, ability to operate at very high frequencies, and under appropriate conditions, the ability to switch AC or DC with equal ease.
The idea of FET switches for relay-type functions and related developments go back to as early as 1955 and have been made since by several skilled laboratories.
In spite of these many efforts going back over two decades, optically activated FET power switches have not proven commercially significant because of their inability to meet the specific cost and performance requirements of certain defined industrial and telecommunications applications.
It is a principal object of the invention, therefore, to provide commercially useful, optically isolated power FET switches capable of
1. operating at low (millivolt) or high (above 300 V) load voltage,
2. operating at low (picoamps) or high (above several amperes) current,
3. switching DC of either polarity or AC up to at least 100 KHz (i.e., capability of sub-microsecond switching speeds),
4. immunity to false latchup (as with thyristors),
5. immunity to secondary breakdown (as with bipolar transistors),
6. switching speeds controllable over wide range (for switching transient control),
7. operating from very low input signals,
8. operating at high temperature,
9. being readily arranged in series or parallel arrays for extended current or voltage capability,
10. duplicating the break-before-make configuration of Form C mechanical relays,
11. exhibiting ON resistance under 100 milliohms and OFF resistance over 10,000 megohms so to approach the isolation of reed relays.